1. Field of Invention
The present invention relates to a method of coating a powder lubricant in a metallic injection molding machine having a die composed of a stationary die and a movable die, a sprue hole, runner groove, gate hole and cavity being formed on parting faces of these dies, molten metal being injected into the cavity via the sprue hole and runner groove so as to fill the cavity with the molten metal. Also, the present invention relates to a die used for metallic injection molding.
2. Related Art
As one of the high pressure casting methods of producing a metallic product from a low melting point metallic material such as aluminum, magnesium and zinc alloy, there is proposed a thixotropic molding method, which is disclosed in Japanese Examined Patent Publication Nos. 1-33541 and 2-15620. According to this method, a part is obtained as follows. When a raw material of alloy is agitated in a state in which solid and liquid are coexisting, the formation of dendrite is suppressed. Therefore, it is possible to obtain a slurry in which fine solid particles and liquid are coexisting. This semi-solid slurry is injected into a die in a short period of time and soldified. In this way, a part of alloy, in the microstructure of which solid is substantially uniformly distributed, can be obtained. Concerning the thus obtained molding, a ratio of contraction caused by solditication is low, and micro-shrinkage is small, that is, the number of trap holes is small and also the number of cavity holes caused by the contamination of gas is small. Therefore, the dimensional accuracy of the thus obtained part is high, and the mechanical property are excellent. An injection molding machine is used for the above method of manufacturing a molding of alloy in which the property of slurry is used. Briefly speaking, the injection molding machine is composed of a heating cylinder and a screw arranged in the heating cylinder, wherein this screw is capable of rotating in the rotational direction and driven in the axial direction. In general, an open type injection nozzle is provided in the front of the heating cylinder. In the outer peripheral sections of the heating cylinder and the injection nozzle, there are provided a plurality of heaters, the heating temperatures of which are individually controlled.
When molten metal is measured by an injection molding machine having an open type injection nozzle, it is necessary to set a plug in order to prevent the molten metal to be measured from leaking out from a forward end portion of the injection nozzle. A cold plug, which is formed when the molten metal is cooled and solidified to some extent, is used for this plug. The cold plug is formed as follows. After the shot, the injection nozzle coming into contact with the die is quickly deprived of heat, that is, heat is quickly conducted to the die. As a result, the molten metal staying at a forward end portion of the injection nozzle is solidified and the cold plug is formed. This cold plug is formed into such hardness that the cold plug can stand the pressure of molten metal, which is being measured, and also the cold plug can be drawn out when the injection pressure is given to it. Accordingly, when the screw is rotated, metallic slurry is successively conveyed to the forward end portion of the barrel. At this time, slurry is melted by the heat given from the heater around the barrel and also by the heat generated by the shearing and the frictional action when the screw is rotated. The thus slurry is stored in the measurement chamber provided in the forward end portion of the heating barrel. At this time, the sprue hole, runner groove and cavity are coated with a powder lubricant, and the screw is driven at high speed in the axial direction. Then, the slurry stored in the measurement chamber is injected into the cavity via the sprue hole, runner groove and gate hole and filled. At this time, the cold plug is caught by the cold plug catcher provided in the movable die. After the slurry has been cooled and solidified, the die is opened. Then, the part can be easily taken out from the die because the powder lubricant is coated inside the die. AT the same time, the sprue and runner are also discharged from the die.
On the other hand, another method of manufacturing a metallic mold is known which is a cold chamber type die casting method. As disclosed in Japanese Unexamined Patent Publication Hei. 6-320246, the die casting machine 50 used for this die casting method is composed as follows. As shown in FIG. 3, the die casting machine 50 includes: a stationary die 52 attached to the stationary base 51; a movable die 53 which composes a pair together with the stationary die 52; a sleeve 54 penetrating the stationary base 51; and a plunger 55 arranged in the sleeve 54 being capable of moving in the axial direction. In the sleeve 54, there is provided a molten metal supply port 56. This molten metal supply port 56 is closed by the shutter 57xe2x80x2 when the powder lubricant is coated. The powder lubricant supply nozzle 57 is attached to this shutter 57xe2x80x2. When molten metal is poured into the sleeve 54, the powder lubricant supply nozzle 57 is retracted together with the shutter 57xe2x80x2. On the parting line between the stationary die 52 and the movable die 53, there is formed a cavity 60 for forming a part. The first exhaust passage 61 is open to an end portion of this cavity 60. The first exhaust passage 61 is opened and closed by the shut-off pin 63 driven by the hydraulic piston cylinder unit 62. The second exhaust passage 64 branches from the first exhaust passage 61 in such a manner that the second exhaust passage 64 makes a right angle with the first exhaust passage 61. Although not shown in FIG. 3, the second exhaust passage 64 is connected with a valve, vacuum tank and exhaust pump via the flexible hose 65. In this connection, the powder lubricant supply nozzle 57 is connected with a powder lubricant supply device, from which a predetermined quantity of powder lubricant is supplied by compressed air, via the flexible hose 58. This powder lubricant supply device is not shown in FIG. 3, either. The ejector plate and ejector pin are not shown in FIG. 3, either.
It is possible to obtain a part as follows by the die casting machine 50 which is composed as described above. As shown in FIG. 3, the movable die 53 is fastened to the stationary die 52, and the shut-off pin 63 is retracted as shown in FIG. 3. Then, the exhaust pump is driven. Then, air is discharged from the cavity 60, gate passage 66, sprue 67 and sleeve 54 into the vacuum tank. Due to the foregoing, the inside of each component is decompressed. At this time, the shutter 57xe2x80x2, to which the powder lubricant supply nozzle 57 is attached, closes the molten metal supply port 56. Due to the foregoing, air is prevented from being sucked from a portion close to the molten metal supply port 56. Powder lubricant is supplied from the powder lubricant supply device by compressed air for a predetermined period of time. Therefore, the powder lubricant adheres onto the inner surfaces of the cavity and others. The shut-off pin 63 is driven by the hydraulic piston cylinder unit 62, so that the first exhaust passage 61 is closed. After that, a predetermined quantity of molten metal is supplied from a crucible into the sleeve 54 via the molten metal supply port 56. The thus supplied molten metal is injected into the cavity 60 by the plunger 55. When the movable die 53 is opened after the thus injected and charged molten metal has been cooled and solidified, the ejector protrudes and a part can be provided.
As described above, according to the metal injection molding machine, it is possible to obtain a part, the dimensional accuracy of which is high and the mechanical property of which is excellent. However, problems may be encountered in the method of coating a powder lubricant. Concerning these problems, there is shown a method of coating a powder lubricant in the above die casting machine 50. Therefore, when this powder lubricant coating method is applied, the problems may be solved. However, according to the powder lubricant coating method of the die casting machine 50, a surface of the cavity 60 is coated with the powder lubricant from the sleeve 54 fixed to the stationary base 51. Therefore, in the case of an injection molding machine, coating is conducted from the sprue hole. In order to conduct coating from the sprue hole, it is necessary to temporarily separate the injection nozzle from the die. In this connection, in order to separate the injection nozzle from the die, the injection unit including the heating barrel, the screw arranged in the heating barrel capable of being driven in the axial and radial direction and the drive unit for driving the screw must be moved each time of coating, that is, each shot. Then, the shot cycle is extended and the productivity is deteriorated. Further, even when the shot cycle is sacrificed to some extent, since the temperature of the die for making a part is about 200xc2x0 C., that is, the temperature of the die for making a part is high, when the powder lubricant supply nozzle is frequently contacted with and separated from the die of high temperature, there is a possibility that the powder lubricant supply nozzle is damaged due to material fatigue. Further, the following problems may be encountered. The powder lubricant is supplied via a flexible hose, and the temperature of this flexible hose becomes relatively high. Therefore, wax contained in the powder lubricant for enhancing the effect of adhesion of the releasing agent to the cavity is melted and adheres to the inside of the hose, which blocks the flow of the powder lubricant in the hose. For the above reasons, it is impossible to apply the method of coating the powder lubricant of the die casting machine 50 to the metal injection molding machine.
The present invention has been accomplished in view of the above circumstances. It is an object of the present invention to provide a method of coating a powder lubricant in a metallic injection molding machine and also it is an object of the present invention to provide a die used for metallic injection molding, characterized in that: it is unnecessary to move the injection unit each time the powder lubricant is coated so that the shot cycle can be shortened and power rates can be reduced. In addition to the above object, it is an object of another invention to provide a method of coating a powder lubricant in a metallic injection molding machine and also it is an object of another invention to provide a die used for metallic injection molding, characterized in that: solid particles of a powder lubricant, which are excellent because there is no possibility of evaporation, are uniformly coated in the cavity.
The above object of the present invention can be accomplished by the structure in which a recess is provided in the movable die and a powder lubricant is supplied form this recess. In the case of a die to which an open type injection nozzle is applied, there is provided a cold plug catcher for receiving a cold plug in the case of injection. Therefore, in order to accomplish the above object, a powder lubricant is supplied form the cold plug catcher. In order to accomplish the above object, the invention provides a method of coating a powder lubricant in a metallic injection molding machine having a die composed of a stationary die and a movable die, a sprue hole, runner groove, gate hole and cavity being formed on parting faces of these dies, a recess communicating with the sprue hole when the movable die is fastened to the stationary die being formed in the movable die, the method of coating a powder lubricant in a metallic injection molding machine comprising the step of coating the powder lubricant from the recess toward the cavity by decompressing the inside of the sprue hole, runner groove, gate hole and cavity from the cavity side. According to the invention, the recess is a cold plug catcher to catch a cold plug in the case of injecting molten metal. The invention provides a die used for metallic injection molding comprising: a stationary die; and a movable die, wherein a sprue hole, runner groove, gate hole and cavity are formed on parting faces of these dies, molten metal is injected into the cavity via the sprue hole and runner groove so as to fill the cavity with the molten metal, a recess communicating with the sprue hole when the movable die is fastened to the stationary die is formed in the movable die, a powder lubricant supply path provided in the movable die is communicated with the recess via a first opening and closing valve, and an exhaust gas path provided in the movable die is communicated with the cavity via a second opening and closing salve. The invention provides a die used for metallic injection molding, wherein the recess is a cold plug catcher, and the first and the second opening and closing valve are shut-off pins respectively arranged in the powder lubricant supply path and the exhaust path being freely moved in the axial direction. The invention provides a die used for metallic injection molding, wherein the exhaust path is communicated with the cavity via an overflow.